C-Methylation of Alcohols, Ketones, and Indoles with Methanol Using

Subscriber access provided by UNIV OF DURHAM

Article

C-Methylation of Alcohols, Ketones, and Indoles with Methanol Using Heterogeneous Platinum Catalysts S. M. A. H. Siddiki, Abeda S. Touchy, Md. A. R. Jamil, Takashi Toyao, and Ken-Ichi Shimizu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04442 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

C-Methylation of Alcohols, Ketones, and Indoles with Methanol Using Heterogeneous Platinum Catalysts

S. M. A. Hakim Siddiki,† Abeda S. Touchy,† Md. A. R. Jamil,† Takashi Toyao,†,‡ Ken-ichi Shimizu*†,‡

†

Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan

‡

Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto

615-8520, Japan

*Corresponding author Ken-ichi Shimizu E-mail: [email protected]

ACS Paragon Plus Environment

1

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

ABSTRACT A versatile, selective, and recyclable heterogeneous catalytic method for the methylation of C–H bonds in alcohols, ketones, and indoles with methanol under oxidant-free conditions using a Pt-loaded carbon (Pt/C) catalyst in the presence of NaOH is reported. This catalytic system is effective for various methylation reactions: (1) the β-methylation of primary alcohols, including aryl, aliphatic, and heterocyclic alcohols, (2) the α-methylation of ketones, and (3) the selective C3-methylation of indoles. The reactions are driven by a borrowing-hydrogen mechanism. The reaction begins with the dehydrogenation of the alcohol(s) to afford aldehydes, which subsequently undergo a condensation reaction with the nucleophile (aldehyde, ketone, or indole), followed by a hydrogenation of the condensation product by Pt–H species to yield the desired product. In all the methylation reactions explored in this study, the Pt/C catalyst exhibits a significantly higher turnover number than other previously reported homogeneous catalytic systems. Moreover, it is demonstrated that the high catalytic activity of Pt can be rationalized in terms of the adsorption energy of hydrogen on the metal surface, as revealed by density functional theory calculations on different metal surfaces. KEYWORDS: heterogeneous catalysis, methylation, borrowing-hydrogen reaction, platinum, density functional theory


2

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1. INTRODUCTION C–C bond formation reactions are of great interest and importance in organic chemistry. Much effort has been devoted especially to the development of atom-efficient and environmentally friendly processes owing to their economic importance and significance in the industrial and academic context.1–4 Nevertheless, many processes still rely on conventional methods that involve cation– anion-based

nucleophilic

organohalides.

5,6

substitution

reactions

between

organometallic

reagents

and

Although these conventional systems are very important, the inevitable workup

procedures, which include hazardous quenching steps, ﬁltrations, and repeated extractions, as well as the concomitant formation of undesirable waste, are problematic, especially on an industrial scale. Another shortcoming of these systems is the use of organometallic agents as homogeneous catalysts. The organometallic agents are often air- and moisture-sensitive, and can thus be difficult to handle.7 From the viewpoint of green and sustainable chemistry, it is highly desirable to develop new processes that do not rely on these toxic and hazardous materials, with the aim to eventually realize ideal yet practical processes.8 Toward this aim, borrowing-hydrogen reactions have attracted much attention in recent years.9– 17

These reactions enable the catalytic formation of not only C–C bonds but also of many other types

of linkages, including C–N bonds, using alcohols as the alkylating reagents, and are thus regarded as an atom-economical manufacturing approach for bulk and fine chemicals.18–24 Among the variety of alcohols used as alkylating reagents, methanol is of particular interest owing to its great potential as a sustainable resource.25 Methanol is currently obtained from natural gas, coal, and biomass. However, in the near future, methanol from CO2 and sustainable H2 will provide a sustainable C1 feedstock for a “methanol economy”, in which methanol will serve as the source material for a wide range of chemical intermediates and products.26–29 It is therefore hardly surprising that great efforts have been devoted to the synthesis of H2, other fuels, and bulk chemicals from methanol.30 However, selective organic transformations of methanol into fine chemicals remain in their infancy, mostly due to the fact that the reactivity of methanol is relatively low compared to that of other conventional methylation reagents such as methyl iodide, dimethyl sulfone, diazomethane, and formaldehyde, which are all considered genotoxic impurities in pharmaceutical manufacturing.31 In addition, the use of methanol as a synthetic building block requires its selective activation for further transformation. This is particularly difficult for heterogeneous catalysts and, therefore, only very few methylation reactions employing methanol for synthetic applications have been reported so far. Recently, substantial progress has been made in borrowing-hydrogen-type catalytic methylation reactions

using

homogeneous

transition-metal

catalysts.32–46

Borrowing-hydrogen-type

C-methylation methods are classified into the following three reaction types: (1) the β-methylation of alcohols,33–35 (2) the α-methylation of ketones,36–41 and (3) the methylation of phenols42,43 and heterocycles,44–46 which includes the C3-methylation of indoles. Methylation methods typically employ homogeneous (or colloidal) Ru, Ir, or Rh catalysts, although all of these suffer from low


3

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

turnover numbers (TONs) and difficulties associated with the catalyst/product separation and the recycling of the catalyst. Another drawback of these methods is their limited synthetic scope. Most methods are applicable only to activated (aromatic) substrates, and a single catalyst that is generally applicable to all the aforementioned C-methylation reactions has not yet been reported. Herein, we report a heterogeneous catalytic system for the three C-methylation processes mentioned above, employing methanol as the methylating agent (Scheme 1). The synthesis is carried out using Pt nanoparticles loaded on carbon (Pt/C) in the presence of NaOH. This is a highly versatile catalytic methylation system that proceeds via a borrowing-hydrogen mechanism. In addition to the development of this catalytic system, we describe the rationalization of the catalytic properties based on density functional theory (DFT) calculations. The observed catalytic activity of a variety of transition-metal catalysts correlates well with the adsorption energy of hydrogen on different metal surfaces. Our results suggest that the adsorption energy of hydrogen can be effectively used as a simple descriptor for C–C bond formation reactions.

Scheme 1. C-methylation reactions employing methanol as the methylating agent. The three types of C-methylation reactions are successfully realized by a Pt-nanoparticles-loaded carbon catalyst (Pt/C).


4

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2. EXPERIMENTAL SECTION General Commercially available organic and inorganic chemicals (Tokyo Chemical Industry, Wako Pure Chemical Industries, Kishida Chemical, or Mitsuwa Chemicals) were used without further purification. Methanol-d3 (CD3OH; 99.8 atom% D) was purchased from Sigma-Aldrich. GC (Shimadzu GC-2014) and GCMS (Shimadzu GCMS-QP2010) analyses were carried out using an Ultra ALLOY capillary column UA+-1 (Frontier Laboratories Ltd.) with N2 or He as the carrier gas. 1H and

13

C NMR spectra

were recorded at ambient temperature on a JEOL-ECX 600 spectrometer operating at 600.17 and 150.92 MHz, respectively, with tetramethylsilane as the internal standard. HAADF-STEM images were taken on a JEM-ARM200F microscope equipped with a JED-2300 EDX spectrometer (JEOL) at an acceleration voltage of 200 kV. The Cs-corrector CESCOR (CEOS) was used in the STEM mode. The samples were prepared by dropping ethanol suspensions of the catalysts on carbon-supported copper grids.

Catalyst Preparation Standard carbon (296 m2 g-1, Kishida Chemical) and SiO2 (Q-10, 300 m2 g-1, Fuji Silysia Chemical Ltd.) supports were obtained commercially. CeO2 (JRC-CEO3, 81 m2 g-1), MgO (JRC-MGO-3), TiO2 (JRC-TIO-4), and H+-exchanged β zeolite (Hβ, SiO2/Al2O3 = 25±5, JRC-Z-HB25) were supplied by the Catalysis Society of Japan. γ-Al2O3 was prepared by calcination of γ-AlOOH (Catapal B Alumina, Sasol) at 900 °C for 3 h. ZrO2 was prepared by hydrolysis of zirconium oxynitrate 2-hydrate in an aqueous solution of NH4OH, followed by filtration, washing with distilled water, drying at 100 °C for 12 h, and finally calcination at 500 °C for 3 h. Nb2O5 was prepared by calcination of Nb2O5nH2O (CBMM) at 500 °C for 3 h. The Pt/C precursor was prepared by the following impregnation method: a mixture of carbon (10 g) and an aqueous HNO3 solution of Pt(NH3)2(NO3)2 that contained 4.96 wt% of Pt (10.62 g) and 50 mL of ion-exchanged water was added to a round-bottom flask (500 mL). The mixture was then stirred (200 rpm) for 15 min at room temperature. Subsequently, the reaction mixture was evaporated to dryness at 50 °C, followed by drying at 90 °C under ambient pressure for 12 h. Prior to each catalytic experiment, a Pt/C sample with 5 wt% Pt loading was prepared by pre-reduction of the precursor in a Pyrex tube under a H2 flow (20 cm3 min-1) at 300 °C for 0.5 h. Other supported Pt catalysts (5 wt% Pt loadings) were prepared by the same method. M/C (M = Rh, Ir, Ru, Pd, Re, Cu, Ni) catalysts with 5 wt% metal loadings were prepared in a similar manner using aqueous HNO3 solutions of Rh(NO3)3 or Pd(NH3)2(NO3)2, or aqueous solutions of metal nitrates (for Ni, Cu), IrCl3nH2O, RuCl3, or NH4ReO4. Typical Procedure for the Catalytic Reactions Pt/C (39 mg; 1 mol% Pt with respect to 1 mmol of substrate) reduced at 300 °C was used as the standard catalyst.47,48 After the reduction, the catalyst was placed in a closed glass tube sealed with


5

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

a septum inlet and cooled to room temperature under H2. Methanol (10 or 20 mmol) was injected through the septum inlet into the glass tube containing the pre-reduced catalyst. Next, the septum was removed under air, and 1 mmol of the substrate (alcohol, ketone, or indol), NaOH (0.1 or 1.5 mmol), n-dodecane (0.25 mmol), and a magnetic stirring bar were added to the tube, which was then introduced in a stainless steel autoclave (28 cm3). After sealing, the autoclave was charged with 1 bar N2 and heated at 100 or 120 °C under stirring (500 rpm). After the reaction, the reactor was cooled to room temperature, before the catalyst was removed by filtration. For the optimization studies (Tables 1 and 2) and the kinetic experiments, the conversions and yields of the products were determined by GC using n-dodecane as the internal standard, while the GC-sensitivity was estimated from commercial compounds or the isolated products. For the substrate scope studies, the products were isolated by column chromatography on silica gel 60 (spherical, 40–100 µm, Kanto Chemical Co. Ltd.) using hexane/ethyl acetate (9:1, v/v) as the eluent, and the yields of the isolated products were determined. The products were identified by 1H and

13

C NMR spectroscopy in

combination with GCMS, whereby the mass spectrometer was equipped with the same column as that used for the GC analyses. For the recycling experiments, after each catalytic cycle, 2-propanol (3 mL) was added to the mixture. The catalyst was separated by centrifugation and washed with water (3 mL) and acetone (6 mL). The catalyst was then dried at 100 °C for 3 h and reduced at 300 °C for 0.5 h under an atmosphere of H2. Subsequently, the reactor was charged with a mixture of the substrate and NaOH and the recovered Pt/C catalyst. It should be noted that the initial formation rates for reactions with yields below 30% were obtained on the basis of the product yield after a reaction time of 2 h for β-methylation of 2-phenylethanol and 1 h for methylation α-methylation of propiophenone and C3-methylation of indole.

Computational Methods All calculations were performed with the Vienna ab-initio simulation package (VASP)49,50 using projector-augmented wave potentials51 and the Perdew–Burke–Ernzerhof (PBE) functional.52 The dispersion-corrected DFT-D2 method was employed to account for van der Waals interactions.53 The lattice constants of the bulk structures were calculated with an energy cut-off of 400 eV and a 15 × 15 × 15 k-point mesh and used in all subsequent calculations. The Ni(111), Cu(111), Rh(111), Pd(111), Ir(111), Pt(111), Ru(0001), and Re(0001) surfaces were modeled by a supercell slab consisting of a 3 × 3 surface unit cell with four atomic layers. It should be noted that the most stable and common planes were used for each metal. The (111) surface (Ni, Cu, Rh, Pd, Ir, and Pt) presents a face-centered-cubic

(fcc)

structure,

while

the

(0001)

surface

(Ru

and

Re)

has

a

hexagonal-close-packed (hcp) structure. The slab was separated in the vertical direction by a vacuum void (height: 15 Å). An energy cut-off of 400 eV and a 5 × 5 × 1 point mesh were used for the slab model calculations. The energy of the isolated H atom was obtained using the same parameters


6

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

as those in the free-surface slab calculations. The geometry-convergence criterion was set at Fmax < 0.03 eV Å-1, where Fmax is the maximum force acting on a mobile atom. Spin-polarized calculations were carried out for all systems. The adsorption energy (Eads) was calculated as the difference in energy between the molecule absorbed on the surface (Emolecule/surface), the individual adsorbate molecule (Emolecule), and the surface (Esurface) according to: Eads = Emolecule/surface – Emolecule + Esurface

(6)

The location of the adsorption sites on the adsorbates was initially determined, which showed that the most stable adsorption sites for hydrogen are the 3-fold hollow sites (Fig. S1; fcc sites for the fcc metals). The adsorption energies employed in this study were calculated for the structures where hydrogen was more strongly adsorbed.


7

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

3. RESULTS AND DISCUSSION Catalyst Characterization A carbon-supported Pt nanoparticles catalyst (Pt/C) was obtained from a facile impregnation method, followed by H2 reduction at 300 °C, according to a previous study using the same Pt/C catalyst.47 Figure 1 shows the annular bright field scanning transmission electron microscopy (ABF-STEM) image and high-angle annular dark-field STEM (HAADF-STEM) images of the Pt/C catalyst together with the corresponding energy-dispersive X-ray spectrometry (EDX) elemental mapping. The Pt nanoparticles are highly dispersed over the entire carbon support. The additional characterization data for Pt/C, e.g., temperature-programmed H2-reduction (H2-TPR) and X-ray absorption fine structure (XAFS) analyses, which have been provided in our previous study, confirmed that the deposited Pt nanoparticles are in the metallic state.47

Figure 1. ABF- and HAADF-STEM images and EDX mapping for Pt/C.


8

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

β-Methylation of Alcohols The β-methylation of 2-phenylethanol (1a) was selected as the benchmark reaction for the β-methylation of alcohols with different heterogeneous catalysts. A catalyst screening was carried out for the reaction of 1a (1 mmol) and methanol (20 mmol) with NaOH (1.5 mmol) at 120 °C for 24 h. It should be noted that the reaction was carried out in a glass reactor that was inserted in a stainless steel autoclave, in which the reactor was heated to temperatures beyond the boiling point of methanol. Initially, we tested a series of Pt-loaded (5 wt%) heterogeneous catalysts (39 mg in total; 1 mol% Pt with respect to 1a). Table 1 lists the yields obtained for 2-phenyl-1-propanol (2a) as the desired product. The reaction did not occur in the absence of a transition-metal catalyst (entry 1), and carbon-supported platinum oxide (PtOx/C) afforded 2a in merely 2% yield (entry 2). The results show that the combination of a base (NaOH) and platinum oxide does not catalyze the reaction. In contrast, carbon loaded with metallic Pt nanoparticles (Pt/C, entry 3) afforded 2a in 91% yield. Exposure of the as-reduced Pt/C catalyst to air at room temperature for 0.5 h resulted in a lower yield of 2a (42%, entry 4), most likely due to air-oxidation of the superficial Pt0 sites, i.e., the active sites of Pt/C. Thus, metal-loaded catalysts reduced under H2 at 300 °C were used for the subsequent reactions in the absence of air (entries 5–19). Pt-metal particles supported on different metal oxides (entries 5–12) generated lower yields of 2a (7–32%) than Pt/C. Subsequently, we compared the activity of various transition metal nanoparticles on the same support (carbon; entries 13–19), whereby the catalytic performance followed the order: Pt > Ir > Rh > Ru > Ni > Pd > Re > Cu. The results in Table 1 indicate that Pt/C (entry 3) is the most suitable catalyst for the β-methylation of 1a in the presence of 1.5 equiv NaOH. The effect of the Pt loading in the Pt/C catalyst was investigated by employing a Pt/C catalyst with 1 wt% of Pt (Table S1). The results show that the Pt/C catalyst with 5 wt% Pt loading exhibited better catalytic performance.


9

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

Table 1. Catalyst screening for the β-methylation of 2-phenylethanol (1a)

Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 a GC yield.

Catalyst None PtOX/C Pt/C Pt/C-air Pt/Al2O3 Pt/CeO2 Pt/MgO Pt/ZrO2 Pt/Nb2O5 Pt/TiO2 Pt/SiO2 Pt/Hβ Rh/C Ir/C Ru/C Pd/C Re/C Cu/C Ni/C

Yield (%)a 0 2 91 42 14 32 11 12 9 8 7 13 28 31 13 10 8 2 11

With the best catalyst in hand, we examined further potential optimizations of the reaction conditions (Table 2). The reaction did not proceed in the absence of a base (entry 1), which indicates that Pt/C alone does not promote the reaction, and that the addition of a base is essential. The results with different additives (entries 2–11) revealed that the efficacy of the base follows the order NaOH > NaOMe > NaOtBu > KOtBu > KOH > NaOEt > Cs2CO3 > Na2CO3 > K2CO3 > Et3N. Catalytic runs with different loadings (0.5–2.0 mmol) of NaOH (entries 2, 12–14) confirmed that 1.5 equiv of NaOH afford the highest yield of 2a. Finally, varying the methanol loading (5–60 mmol, entries 2, 15– 18) revealed the optimal amount of methanol (20 mmol).


10

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 2. Optimization of the reaction conditions for the β-methylation of 2-phenylethanol

Entry CH3OH (mmol) 1 20 2 20 3 20 4 20 5 20 6 20 7 20 8 20 9 20 10 20 11 20 12 20 13 20 14 20 15 5 16 10 17 40 18 60 a GC yield.

Base (x) None NaOH (1.5) KOH (1.5) Na2CO3 (1.5) K2CO3 (1.5) Cs2CO3 (1.5) NaOtBu (1.5) KOtBu (1.5) NaOMe (1.5) NaOEt (1.5) Et3N (1.5) NaOH (0.5) NaOH (1.0) NaOH (2.0) NaOH (1.5) NaOH (1.5) NaOH (1.5) NaOH (1.5)

Yield (%)a 0 91 54 17 13 34 64 67 70 49 0 28 74 85 74 83 90 52

Under the optimized conditions, we studied the catalytic performance of the Pt/C-catalyzed β-methylation of various alcohols with methanol. Table 3 shows the scope of primary alcohols for the present catalytic β-methylation system. For all the substrates except the linear aliphatic alcohols (entries 14 and 15), full conversion of the alcohol was observed. The yields of the isolated β-methylation products are listed in Table 3. 2-Phenylethanol (entry 1) and its derivatives with electron-donating (entries 2–4) and electron-withdrawing (entry 5) groups at the para-position were successfully methylated to give the corresponding β-methylated alcohols in good to high isolated yield (76–87%). 2-Naphthalene-1-yl ethanol (entry 6), 3-phenylpropan-1-ol (entry 7), and 3-(4-bromophenyl)propan-1-ol (entry 8) afforded methylated alcohols in good to high yield. Heteroaromatic alcohols with indole-3-propyl (entry 9), pyridinyl (entry 10), and thienyl (entry 11) groups, or with a 2-(benzyloxy)ethan-1-ol moiety (entry 12) were well tolerated and furnished the corresponding β-methylated alcohols in high yield (83–95%). Aliphatic primary alcohols (entries 13– 15) also underwent this β-methylation reaction to afford the corresponding products in good yield (73–85%), thus demonstrating the high versatility of this heterogeneous catalytic system for the β-methylation of aliphatic alcohols with methanol.


11

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

Table 3. Pt/C-catalyzed β-methylation of primary alcohols

t (h)

Yield (%)a

24

87

24

78

24

81

30

78

24

76

36

88

24

92

24

73

12

95

24

83

11

24

88

12

24

86

24

85

14

30

77

15

30

73

Entry

Alcohol

Product OH

1

OH

2 H3C

OH

3 H3CO

4 OH

5 F

OH

6 OH

7

OH

8

Br

OH

9

10

13

a

N H OH N

OH

Isolated yield.


12

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Recently, Obora and co-workers have reported the first example of the β-methylation of secondary alcohols with methanol.34 Equation (1) shows the results for the β-methylation of secondary alcohols by our method using 1 mol% of Pt/C catalyst with 1.5 equiv of NaOH at 120 °C for 24 h. 1-Phenylethanol and its derivatives with electron-donating (methyl, methoxy) groups at the para-position were converted into bis-methylated products (alcohols and ketones) in 83–88% yield. In

the

literature,

only

one

homogeneous

catalyst,33

i.e.,

a

combination

of

carbonylchlorohydrido{bis[2-(diphenylphosphinomethyl)ethyl]amino}ethyl]amino}Ru(II) (Ru-MACHO) and Shvo’s diruthenium complex, and one heterogeneous catalyst,34 i.e., colloidal Ir nanoclusters, have been reported to catalyze the β-methylation of a variety of alcohols with methanol. Table 4 compares the catalytic performance of our system to that of these previously reported catalysts. While the previous methods are effective only for activated substrates, i.e., aromatic alcohols, our method is effective for aromatic and aliphatic (non-activated) alcohols. As shown in equation (2), a gram-scale reaction of 1a (5 mmol) with 20 equiv of methanol and 1.5 equiv of NaOH using 0.025 mol% of the Pt/C catalyst for 90 h afforded 2a in high yield (82%). This value corresponds to a TON of 3280, which is one order of magnitude higher than those of the previous methods for the β-methylation of 1a.33,34

Table 4. β-Methylation of alcohols with methanol using different catalysts Substrate scope TONa Catalyst reuse Ref. 2-Arylethanols 3-Arylpropanols Pt/C 3280 5 runs This work Aliphatic alcohols 1-Phenylethanol Ru complex 2-Arylethanols 435 No 33 2-Arylethanols Ir nanocluster 356 No 34 1-Phenylethanol a TON per number of total metal atoms in the catalysts for the β-methylation of 2-phenylethanol. Catalyst


13

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

Next, we examined the heterogeneous nature and reusability of the Pt/C catalyst. After a standard reaction (entry 1 in Table 3), 2-propanol (3 mL) was added to the mixture and Pt/C was removed by filtration, followed by inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis of the reaction mixture, which showed that the content of Pt in the solution was below the detection limit (10 ppb). After a standard workup and catalyst recovery,54 the spent catalyst was reused four times, whereby a significant loss of activity was not observed (Fig. 2). In addition to the final yield, the initial product formation rates were investigated for each cycle, and the results are shown in Fig. S2 in the Supporting Information. These results do not show a significant decrease in activity, which confirms the high durability of the catalyst. The necessity of H2 reduction prior to each recycling test was examined (Table S2). The yield for the second run decreased when the catalyst was used without prior H2 reduction, which indicates that H2 reduction is indispensable for the recycling the catalyst. These results unequivocally demonstrate that Pt/C is a reusable heterogeneous catalyst for this reaction. Considering the previously proposed mechanisms for such reactions,33,34 our β-methylation method may likely follow a borrowing-hydrogen mechanism (Scheme 2). To examine this hypothesis, we carried out the following kinetic and mechanistic experiments. Figure 3 shows the time course of the products (GC analysis) for the Pt/C-catalyzed β-methylation of 2-naphthalene-1-yl ethanol (1b). Initially (t < 5 h), 2-(naphthalen-1-yl)acetaldehyde (4b) is generated, although its content decreases with time. For 5 h < t < 35 h, the molar fraction of 2-(naphthalen-1-yl)acrylaldehyde (3b) initially increases, and then decreases. Finally (t > 35 h), the amount of 2-naphthalene-1-yl propanol (2b) reaches its highest value. This is a time course typical for consecutive reactions. Aldehyde 4b represents the initial intermediate, which is subsequently transformed into the α,β-unsaturated aldehyde intermediate 3b, which serves as the precursor for the hydrogenated product 2b. Figure 4 shows the results of the kinetic isotope effect (KIE) analysis for the β-methylation reaction of the standard substrate 2-phenylethanol (1a). The zero-order rate constant for CH3OH (kCH), estimated from the initial slope of the plot, is larger than that for CD3OH (kCD), and a moderate KIE (kCH/kCD) value of 1.8 was obtained. This suggests that the catalytic cycle involves the dissociation of a C–H bond in methanol as a kinetically important step. As shown in eqn (3), the cross-aldol

condensation

of

the

dehydrogenated

intermediates

(formaldehyde

and

phenylacetaldehyde) was catalyzed by 0.5 equiv of NaOH (in the absence of Pt/C) under N2 at a relatively low temperature (80 °C). In the presence of Pt/C and NaOH at 120 °C, the reaction of formaldehyde and phenylacetaldehyde under H2 furnished 2-phenyl-1-propanol (2a), the final product for the standard β-methylation of 2-phenylethanol (1a) with methanol.


14

Page 15 of 31

O

CH2

O

0.5 equiv NaOH

+

O

H H 1 mL toluene (paraformaldehdye) 80 °C, 12 h, N 2 20 mmol

1 mmol

O H

20 mmol

1 mmol

CH3 OH

0.5 equiv NaOH 1 atm H2

H

(3)

63% yield

1 mol% Pt/C

O +

(4)

46% yield

120 °C, 12 h

Based on these mechanistic results, we conclude that this catalytic system is driven by a borrowing-hydrogen mechanism (Scheme 2). Both alcohols, i.e., the starting reagent and methanol are dehydrogenated to give the corresponding aldehydes and four atoms of hydrogen on the catalyst surface. The aldehydes undergo a cross-aldol condensation to generate an α,β-unsaturated aldehyde intermediate, which is then reduced by the four hydrogen atoms on the Pt surface. 100 80 GC yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

60 40 20 0

1

2

3

4

5

Cycle number

Figure 2. Catalyst recyclability for the β-methylation of 2-phenylethanol (1a) with methanol and the Pt/C catalyst. The conditions used are those of entry 1 in Table 3.

Figure 3. Reaction course for (1b) 2-naphthalene-1-yl ethanol, (2b) 2-naphthalene-1-yl propanol, (3b) 2(naphthalen-1-yl)acrylaldehyde, and (4b) 2-(naphthalen-1-yl)acetaldehyde.


15

ACS Catalysis

Scheme 2. Proposed catalytic cycle for the β-methylation of alcohols.

CH3OH

0.3 Product / mmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

CD3OH

0.2

kCH / kCD = 1.8 0.1

0

1

2

3 t/h

4

5

Figure 4. Kinetic isotopic effect analysis for the Pt/C-catalyzed β-methylation of 1b under the conditions shown in Fig. 3. The KIE (kCH/kCD) value estimated from the zero-order rate constants (the slope of the lines) is shown.


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

α-Methylation of Ketones The same catalytic system, i.e., Pt/C and NaOH, is also effective for the α-methylation of ketones with methanol. In the literature, several homogeneous catalysts have been reported to catalyze this reaction, but the known methods suffer from drawbacks, usually associated with scope limitations and/or low TONs.36–41 Here, we present a reusable catalyst with wide scope and high TONs. Initially, we carried out the optimization of the catalyst and reaction conditions for the α-methylation of propiophenone. Based on the results (not shown), we adopted the following optimized reaction conditions: ketone (1 mmol), methanol (10 mmol), 1 mol% of Pt/C, 10 mol% NaOH, 100 °C, and 15 h. Table 5 summarizes the general applicability of the present system for the α-methylation of aromatic ketones. The examined aromatic and aliphatic ketones are mainly mono-methylated at the α-position. Propiophenone (entry 1), butyrophenone (entry 2), benzyl phenyl ketones (entries 3 and 4), other benzyl ketones (entries 5 and 6), and linear and cyclic aliphatic ketones (entries 7–10) can be converted into the corresponding α-methylation products in good to high yield (73–94%). Note that doubly methylated compounds were observed as minor products for entries 7, 8, and 10. Figure 5 shows the results for the catalyst recyclability in the α-methylation of propiophenone under the standard conditions shown in Table 5 (entry 1). Using the standard method for the catalyst separation and regeneration,54 the Pt/C catalyst was reused four times, demonstrating that Pt/C is a robust heterogeneous catalyst for this reaction. The examination of the initial product formation rates for each cycle confirms the high durability of the catalyst (Figure S3). Table 6 lists the results for the α-methylation of methyl ketones and aliphatic ketones with methanol. Acetophenone (entry 1) and its derivatives with electron-donating (entries 2 and 3) and electron-withdrawing (entry 4) groups were selectively bis-methylated, while mono-methylated products were not observed by GC analysis. Aliphatic ketones (entries 5 and 6), including linear and branched ketones, were predominantly mono-methylated at the terminal position, albeit that minor amounts of 1,3-dimethylated (14% and 18%) and 1,1,3-trimethylated (8% and 7%) products were also obtained.


17

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

Table 5. Pt/C-catalyzed α-methylation of aromatic and aliphatic ketones.

Entry

Ketone

Methylated product(s)

Yield (%)a

1

90

2

88

3

94

4

85

5

89

6

87 O

7

73b, 19b , 74b, 16b

8 ,

88b

9

10 a

,

93, 3b

Isolated yield, b GC yield.


18

Page 19 of 31

100 GC yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80 60 40 20 0

1

2

3

4

5

Cycle number

Figure 5. Catalyst recyclability for the α-methylation of propiophenone with methanol and the Pt/C catalyst. The conditions used are those described in Table 5 (entry 1). Table 6. Pt/C-catalyzed α-methylation of methyl ketones and aliphatic ketones. O R1 1 mmol

Entry

+

CH3OH 10 mmol

O

1 mol% Pt/C 0.1 equiv NaOH 100 °C, 15 h

Ketone

R

1

O CH3

or

R1

CH 3

CH 3

Methylated product(s)

Yield (%)a

1

91

2

93

3

88

4

79

5

76b, 14b, 8b ,

, 71b, 18b, 7b

6 , a

,

Isolated yield, b GC yield.


19

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

The method was also successfully applied to the gram-scale methylation of several ketones. Scheme 3 summarizes the isolated yields of the products for the α-methylation of 10 mmol of various ketones for 90 h using only 0.025 mol% of the Pt/C catalyst. Acetophenone was transformed into isobutyrophenone in 84% yield. A variety of aromatic and aliphatic ketones were converted into the corresponding mono-methylated products in 78–86% yield. The TON values for these reactions were >3000. Several homogeneous catalysts, such as [Cp*RhCl2]2, [Cp*RuCl2]2, and [Cp*IrCl2]2 catalyze the α-methylation of ketones with methanol.36–41 Table 7 compares the performance of our method to that of the representative catalysts [Cp*RhCl2]2 and [Cp*IrCl2]2 in the α-methylation of ketones. Again, these homogeneous catalytic systems are only effective for activated (aromatic) ketones, while our method is effective for aromatic and aliphatic (non-activated) ketones. In the α-methylation of 2-propiophenone, the TON of the Pt/C catalyst (3440) is two orders of magnitude higher than those of the previous methods for the same reaction.39,40 Based on the mechanistic results for the β-methylation of alcohols, a reasonable catalytic cycle for this reaction is proposed in Scheme 4. Methanol is dehydrogenated to give formaldehyde and two atoms of hydrogen on the catalyst surface. Formaldehyde and the ketone substrate then undergo a cross-aldol condensation reaction to give an α,β-unsaturated intermediate, which is finally reduced by the two hydrogen atoms on the Pt surface.

Scheme 3. Gram-scale α-methylation of ketones. Table 7. α-Methylation of ketones with methanol using different catalysts Catalyst Additives TON a Catalyst reuse Ref. Pt/C 0.1 equiv NaOH 3440 5 runs This work Rh complex 5 equiv Cs2CO3, O2 15 No 36 Ir complex 4 equiv Cs2CO3 95 No 40 a TON per number of total metal atoms in the catalysts for the α-methylation of 2-propiophenone.

Scheme 4. Proposed catalytic cycle for the α-methylation of ketones.


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

C3-Methylation of Indoles The developed catalytic system (Pt/C and NaOH) was furthermore applied to the C3-methylation of indoles with methanol. Only one homogeneous catalytic method has so far been reported for this reaction, but the method suffers from low TONs as well as difficulties associated with the removal and recycling of the catalyst.45 Thus, the development of more efficient and reusable heterogeneous catalysts is highly desirable. Preliminary experiments for the optimization of this reaction revealed the optimum conditions: indole (1 mmol), methanol (10 mmol), Pt/C (1 mol%), NaOH (10 mol%), 100 °C, and 14 h. Under these optimized conditions, the C3-methylation of a variety of indoles afforded the desired products (Table 8). The reactions of indole (entry 1), C5-substituted indoles with electron-donating and electron-withdrawing groups (entries 2–5), 2,5-dimethylindole (entry 6), and 2-methyl-indole (entry 7) proceeded smoothly to give the corresponding C3-methylated products in high yield (81–95%). 3-Methyl indole (entry 8) and N-methyl indole (entry 9) did not undergo such a C3-methylation. This result is consistent with a proposed reaction route (Scheme 5) that involves a nucleophilic attack of the C3-carbon of the indole to the formaldehyde molecule. It should be noted that neither 3,3’-bisindolylmethane nor its derivatives are generated by this method, which supports the borrowing-hydrogen mechanism depicted in Scheme 5. Catalyst recycling tests were carried out for the C3-methylation of indole using the standard method for catalyst separation and regeneration.54 As shown in Figure 6, the spent Pt/C catalyst was reused at least four times, which demonstrates that Pt/C is a robust heterogeneous catalyst for this reaction. The initial product formation rates were examined for each cycle in addition to the final yields (Fig. S4). A significant loss of the initial rate was not observed, which suggests that Pt/C is a robust heterogeneous catalyst. Using a low Pt/C loading (0.025 mol%), a gram-scale reaction of 5 mmol indole with 50 mmol methanol at 100 °C for 90 h resulted in a high isolated yield of the product (88%), corresponding to a TON of 3520 (eqn (5)). Table 9 compares the main features of the present method with those of the previously reported system with a homogeneous [Cp*IrCl2]2 catalyst for the C3-methylation of indole. Our method is superior to the reported one in terms of the TON (two orders of magnitude higher) and catalyst reusability. These results demonstrate that this heterogeneous catalytic system, which is based on Pt/C and NaOH, is highly versatile for C-methylation transformations, including the β-methylation of primary alcohols, the α-methylation of ketones, and the C3-methylation of indoles. Importantly, all these reactions use methanol as a sustainable C1 source, and additional hydrogen sources are not required.


21

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

Table 8. Pt/C-catalyzed C3-methylation of indoles

Entry

Indole

Product

Yield (%)a

CH3

1

95 N H

N H

2

88

3

85

4

83

5

81b H3C

CH3

6 N H

a

CH3

86

7

93

8

0

9

0

Isolated yield, bGC yield

Scheme 5. Proposed catalytic cycle for the C3-methylation of indole.


22

Page 23 of 31

100 GC yield / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80 60 40 20 0

1

2

3

4

5

Cycle number

Figure 6. Catalyst recyclability for the C3-methylation of indole with methanol and the Pt/C catalyst. The conditions used are those described in Table 8 (entry 1). Table 9. Catalyst comparison for the C3-methylation of indole Catalyst Additives TON a Catalyst reuse Pt/C 0.1 equiv NaOH 3520 5 runs 1 equiv KOtBu Ir complex 24 No a TON per number of total metal atoms in the catalysts.


Ref. This work 45

23

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

Relationship between the catalytic activity and the adsorption energy of hydrogen: Volcano plots Rationalizing the properties of catalysts and establishing theories for heterogeneous catalysis are challenging but inevitable tasks in order to understand the underlying phenomena and ultimately develop improved catalysts.55,56 Recent findings in this area, obtained from density functional theory (DFT) calculations, have shown that the adsorption energy of atoms, molecules, and related species can serve as a simple descriptor to rationalize the activity and selectivity of heterogeneous catalysts. 57-60

This is based on the knowledge that adsorption is a fundamental step for surface-catalyzed

reactions following the Brønsted–Evans–Polanyi (BEP) relation.61,62 It is also well-known that a Sabatier-type trade-oﬀ exists between the adsorption of reactants and the desorption of products, so that catalysts that exhibit optimal adsorption energies afford maximal catalytic activities.63,64 Therefore, volcano plots are usually obtained for catalytic processes. This rationalization has been applied to relatively simple catalytic reactions involving gas molecules as products, but not extensively to organic transformation reactions.65 Here, we disclose that the adsorption energy of hydrogen on different metal surfaces correlates well with the catalytic activity of the metals in methylation reactions. Atomic hydrogen was chosen as the adsorbate, as methylation reactions are considered to typically proceed via a hydrogen-borrowing mechanism, implying that adsorbed hydrogen species are involved in the catalytic cycle. The adsorption energy of H on Ni(111), Cu(111), Rh(111), Pd(111), Ir(111), Pt(111), Ru(0001), and Re(0001) surfaces was initially determined theoretically by means of DFT calculations, and subsequently employed as descriptors to explain the catalytic activity of different catalysts (see the experimental section for computational details and the Supporting Information for the optimized structures). Figure 7 plots the reaction rates (per number of total metal atoms in the catalyst) for the β-methylation of 2-phenylethanol, the α-methylation of 2-propiophenone, and the C3-methylation of indole, over a series of metal-loaded carbon catalysts as a function of the hydrogen adsorption energy. Platinum, which is the most active metal tested, displays a moderate hydrogen adsorption energy, and volcano-type correlations are observed for all three reactions. These observations indicate that the adsorption energy of hydrogen on metal surfaces serves as a good descriptor to explain the activity of a catalyst in certain organic transformation processes. This finding should thus contribute to the rationalization of the catalytic activity of metals in hydrogen-borrowing reactions and to the future design of catalysts without the need for extensive trial-and-error experimental testing.


24

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 7. (A) Adsorption models for H on the Pt(111) surface; top (left) and side (right) view. Reaction rates (based on the total number of metal atoms in the catalyst) for (B) the β-methylation of 2-phenylethanol, (C) the α-methylation of 2-propiophenone, and (D) the C3-methylation of indole by metal(M)-loaded carbon catalysts as a function of the adsorption energy of hydrogen on the metal surface.


25

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

CONCLUSIONS Highly versatile, selective, and recyclable heterogeneous catalytic processes were developed for three C-methylation reactions using methanol as a sustainable C1 source: (1) the β-methylation of primary alcohols, (2) the α-methylation of ketones, and (3) the C3-methylation of indoles. These catalytic

systems

are

driven

by

a

borrowing-hydrogen

mechanism,

wherein

the

Pt-nanoparticles-catalyzed dehydrogenation of methanol (and alcohols) to aldehydes is followed by a condensation of formaldehyde with the corresponding nucleophile (aldehyde, ketone, or indole) to yield unsaturated intermediates, which are subsequently hydrogenated by Pt–H species on the catalyst. Compared to previous catalytic methods for borrowing-hydrogen-type C-methylation reactions, our method presents the following advantages: (1) easy catalyst/product separation, (2) catalyst recyclability, (3) excellent TONs (up to two orders of magnitude higher than those previously reported), and (4) wide substrate scope. Furthermore, DFT calculations have shown that the adsorption energy of hydrogen correlates well with the catalytic activity of various active metals employed in these reactions. These results should be helpful to rationalize the properties of catalytic systems, and serve as a practical guide for future catalyst design. AUTHOR INFORMATION

Corresponding author Ken-ichi Shimizu E-mail: [email protected]

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. A representative structure model for DFT calculations, results of recycling studies, and NMR data. ACKNOWLEDGEMENT This work was supported by JSPS KAKENHI grant 17H01341 and 16H06595 from the Japan Society for the Promotion of Science (JSPS) and by the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT) within the projects "Integrated Research Consortium on Chemical Sciences (IRCCS)" and "Elements Strategy Initiative to Form Core Research Center", as well as by the JST-CREST project JPMJCR17J3. In this research work we used the supercomputer of ACCMS, Kyoto University. The authors thank Ms. Naomi Hirai, the Open Facility of Hokkaido University, for her help with STEM analysis and are indebted to the technical division of the Institute for Catalysis (Hokkaido University) for the manufacturing of experimental equipment.


26

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

REFERENCES (1)

Jia, C.; Kitamura, T.; Fujiwara, Y. Catalytic Functionalization of Arenes and Alkanes via C–H Bond Activation. Acc. Chem. Res. 2001, 34, 633–639.

(2)

Song, G.; Wang, F.; Li, X. C–C, C–O and C–N Bond Formation via Rhodium(iii)-Catalyzed Oxidative C– H Activation. Chem. Soc. Rev. 2012, 41, 3651.

(3)

Bariwal, J.; Van der Eycken, E. C–N Bond Forming Cross-Coupling Reactions: An Overview. Chem. Soc. Rev. 2013, 42, 9283.

(4)

Satoh, T.; Miura, M. Oxidative Coupling of Aromatic Substrates with Alkynes and Alkenes under Rhodium Catalysis. Chem.- Eur. J. 2010, 16, 11212–11222.

(5)

Huang, F.; Liu, Z.; Yu, Z. C-Alkylation of Ketones and Related Compounds by Alcohols: Transition-Metal-Catalyzed Dehydrogenation. Angew. Chemie Int. Ed. 2016, 55, 862–875.

(6)

Obora, Y. C-Alkylation by Hydrogen Autotransfer Reactions. Top. Curr. Chem. 2016, 374, 11.

(7)

Astruc, D. Organometallic chemistry and catalysis; Springer: Berlin, Heidelberg, 2007.

(8)

Sawama, Y.; Asai, S.; Monguchi, Y.; Sajiki, H. Versatile Oxidation Methods for Organic and Inorganic Substrates Catalyzed by Platinum-Group Metals on Carbons. Chem. Rec. 2016, 16, 261–272.

(9)

Shimizu, K. Heterogeneous Catalysis for the Direct Synthesis of Chemicals by Borrowing Hydrogen Methodology. Catal. Sci. Technol. 2015, 5, 1412–1427.

(10)

Yang, Q.; Wang, Q.; Yu, Z. Substitution of Alcohols by N-Nucleophiles via Transition Metal-Catalyzed Dehydrogenation. Chem. Soc. Rev. 2015, 44, 2305–2329.

(11)

Muzart, J. Pd-Catalyzed Hydrogen-Transfer Reactions from Alcohols to C=C, C=O, and C=N Bonds. European J. Org. Chem. 2015, 2015, 5693–5707.

(12)

Gunanathan,

C.;

Milstein,

D.

Applications

of

Acceptorless

Dehydrogenation

and

Related

Transformations in Chemical Synthesis. Science 2013, 341, 1229712–1229712. (13)

Bähn, S.; Imm, S.; Neubert, L.; Zhang, M.; Neumann, H.; Beller, M. The Catalytic Amination of Alcohols. ChemCatChem 2011, 3, 1853–1864.

(14)

Guillena, G.; Ramón, D. J.; Yus, M. Hydrogen Autotransfer in the N-Alkylation of Amines and Related Compounds Using Alcohols and Amines as Electrophiles. Chem. Rev. 2010, 110, 1611–1641.

(15)

Guillena, G.; Ramón, D. J.; Yus, M. Alcohols as Electrophiles in C–C Bond-Forming Reactions: The Hydrogen Autotransfer Process. Angew. Chem. Int. Ed. 2007, 46, 2358–2364.

(16)

Nixon, T. D.; Whittlesey, M. K.; Williams, J. M. J. Transition Metal Catalysed Reactions of Alcohols Using Borrowing Hydrogen Methodology. Dalt. Trans. 2009, 5, 753–762.

(17)

Hamid, M. H. S. A.; Slatford, P. A.; Williams, J. M. J. Borrowing Hydrogen in the Activation of Alcohols. Adv. Synth. Catal. 2007, 349, 1555–1575.

(18)

Fujita, K.; Li, Z.; Yamaguchi, R. N-Alkylation of Amines with Alcohols Catalyzed by a Cp* Ir Complex. Tetrahedron Lett. 2003, 44, 2687–2690.

(19)

Yamaguchi, K.; He, J.; Oishi, T.; Mizuno, N. The “borrowing Hydrogen Strategy” by Supported Ruthenium Hydroxide Catalysts: Synthetic Scope of Symmetrically and Unsymmetrically Substituted Amines. Chem.- Eur. J. 2010, 16, 7199–7207.

(20)

Li, F.; Xie, J.; Shan, H.; Sun, C.; Chen, L. General and Efficient Method for Direct N-Monomethylation of


27

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

Aromatic Primary Amines with Methanol. RSC Adv. 2012, 2, 8645. (21)

Campos, J.; Sharninghausen, L. S.; Manas, M. G.; Crabtree, R. H. Methanol Dehydrogenation by Iridium N-Heterocyclic Carbene Complexes. Inorg. Chem. 2015, 54, 5079–5084.

(22)

Dang, T. T.; Ramalingam, B.; Seayad, A. M. Efficient Ruthenium-Catalyzed N-Methylation of Amines Using Methanol. ACS Catal. 2015, 5, 4082–4088.

(23)

Neumann, J.; Elangovan, S.; Spannenberg, A.; Junge, K.; Beller, M. Improved and General Manganese-Catalyzed N-Methylation of Aromatic Amines Using Methanol. Chem.- Eur. J. 2017, 23, 5410–5413.

(24)

Paul, B.; Shee, S.; Chakrabarti, K.; Kundu, S. Tandem Transformation of Nitro Compounds into N-Methylated Amines: Greener Strategy for the Utilization of Methanol as a Methylating Agent. ChemSusChem 2017, 10, 2370–2374.

(25)

Goeppert, A.; Czaun, M.; Jones, J.-P.; Surya Prakash, G. K.; Olah, G. A. Recycling of Carbon Dioxide to Methanol and Derived Products – Closing the Loop. Chem. Soc. Rev. 2014, 43, 7995–8048.

(26)

Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy, 2nd ed.; Wiley-VCH: Weinheim, 2009.

(27)

Natte, K.; Neumann, H.; Beller, M.; Jagadeesh, R. V. Transition-Metal-Catalyzed Utilization of Methanol as a C1 Source in Organic Synthesis. Angew. Chem. Int. Ed. 2017, 56, 6384–6394.

(28)

Alberico, E.; Nielsen, M. Towards a Methanol Economy Based on Homogeneous Catalysis: Methanol to H2 and CO2 to Methanol. Chem. Commun. 2015, 51, 6714–6725.

(29)

Maeda, K.; Domen, K. Development of Novel Photocatalyst and Cocatalyst Materials for Water Splitting under Visible Light. Bull. Chem. Soc. Jpn. 2016, 89, 627–648.

(30)

Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Chemical Recycling of Carbon Dioxide to Methanol and Dimethyl Ether: From Greenhouse Gas to Renewable, Environmentally Carbon Neutral Fuels and Synthetic Hydrocarbons. J. Org. Chem. 2009, 74, 487–498.

(31)

Szekely, G.; De Sousa, M. C. A.; Gil, M.; Ferreira, F. C.; Heggie, W. Genotoxic Impurities in Pharmaceutical Manufacturing: Sources, Regulations, and Mitigation. Chem. Rev. 2015, 115, 8182– 8229.

(32)

Natte, K.; Neumann, H.; Beller, M.; Jagadeesh, R. V. Transition-Metal-Catalyzed Utilization of Methanol as a C1 Source in Organic Synthesis. Angew. Chem. Int. Ed. 2017, 55, 6384–6394.

(33)

Li, Y.; Li, H.; Junge, H.; Beller, M. Selective Ruthenium-Catalyzed Methylation of 2-Arylethanols Using Methanol as C1 Feedstock. Chem. Commun. 2014, 50, 14991–14994.

(34)

Oikawa, K.; Itoh, S.; Yano, H.; Kawasaki, H.; Obora, Y. Preparation and Use of DMF-Stabilized Iridium Nanoclusters as Methylation Catalysts Using Methanol as the C1 Source. Chem. Commun. 2017, 53, 1080–1083.

(35)

Wingad, R. L.; Bergström, E. J. E.; Everett, M.; Pellow, K. J.; Wass, D. F. Wingad, R. L.; Bergström, E. J. E.; Everett, M.; Pellow, K. J.; Wass, D. F. Catalytic Conversion of Methanol/ethanol to Isobutanol – a Highly Selective Route to an Advanced Biofuel. Chem. Commun. 2016, 52, 5202–5204.

(36)

Li, F.; Ma, J.; Wang, N. α-Alkylation of Ketones with Primary Alcohols Catalyzed by a Cp∗Ir Complex Bearing a Functional Bipyridonate Ligand. J. Org. Chem. 2014, 79, 10447–10455.

(37)

Shen, D.; Poole, D. L.; Shotton, C. C.; Kornahrens, A. F.; Healy, M. P.; Donohoe, T. J.


28

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Hydrogen-Borrowing and Interrupted-Hydrogen-Borrowing Reactions of Ketones and Methanol Catalyzed by Iridium. Angew. Chem. Int. Ed. 2015, 54, 1642–1645. (38)

Ogawa, S.; Obora, Y. Iridium-Catalyzed Selective α-Methylation of Ketones with Methanol. Chem. Commun. 2014, 50, 2491–2493.

(39)

Chan, L. K. M.; Poole, D. L.; Shen, D.; Healy, M. P.; Donohoe, T. J. Rhodium-Catalyzed Ketone Methylation Using Methanol under Mild Conditions: Formation of α-Branched Products. Angew. Chem. Int. Ed. 2014, 53, 761–765.

(40)

Quan, X.; Kerdphon, S.; Andersson, P. G. C–C Coupling of Ketones with Methanol Catalyzed by a N-Heterocyclic Carbene-Phosphine Iridium Complex. Chem.-Eur. J. 2015, 21, 3576–3579.

(41)

Dang, T. T.; Seayad, A. M. A Convenient Ruthenium-Catalysed α-Methylation of Carbonyl Compounds Using Methanol. Adv. Synth. Catal. 2016, 358, 3373–3380.

(42)

Kim, S.; Hong, S. H. Ruthenium-Catalyzed Aminomethylation and Methylation of Phenol Derivatives Utilizing Methanol as the C1 Source. Adv. Synth. Catal. 2017, 359, 798–810.

(43)

Mastalir, M.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Manganese-Catalyzed Aminomethylation of Aromatic Compounds with Methanol as a Sustainable C1 Building Block. J. Am. Chem. Soc. 2017, 139, 8812–8815.

(44)

Sun, C.; Zou, X.; Li, F. Direct Use of Methanol as an Alternative to Formaldehyde for the Synthesis of 3,3'-Bisindolylmethanes (3,3'-BIMs). Chem.-Eur. J. 2013, 19, 14030–14033.

(45)

Chen, S.-J.; Lu, G.-P.; Cai, C. Iridium-Catalyzed Methylation of Indoles and Pyrroles Using Methanol as Feedstock. RSC Adv. 2015, 5, 70329–70332.

(46)

Jin, J.; MacMillan, D. W. C. Alcohols as Alkylating Agents in Heteroarene C–H Functionalization. Nature 2015, 525, 87–90.

(47)

Hakim Siddiki, S. M. A.; Touchy, A. S.; Kon, K.; Shimizu, K. I. Direct Olefination of Alcohols with Sulfones by Using Heterogeneous Platinum Catalysts. Chem.-Eur. J. 2016, 22, 6111–6119.

(48)

Siddiki, S. M. A. H.; Touchy, A. S.; Chaudhari, C.; Kon, K.; Toyao, T.; Shimizu, K. Synthesis of 2,5-Disubstituted Pyrroles via Dehydrogenative Condensation of Secondary Alcohols and 1,2-Amino Alcohols by Supported Platinum Catalysts. Org. Chem. Front. 2016, 3, 846–851.

(49)

Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186.

(50)

Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50.

(51)

Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979.

(52)

Perdew, J.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868.

(53)

Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787–1799.

(54)

After each cycle, 2-propanol (3 mL) was added to the mixture and the catalyst was separated by centrifugation, followed by washing with water (3 mL) and acetone (6 mL), drying at 100 °C for 3 h, and by H2-reduction at 300 °C for 0.5 h. Then, the reactor was charged with a mixture of the substrate and NaOH and the recovered Pt/C catalyst.


29

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(55)

Page 30 of 31

Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the Computational Design of Solid Catalysts. Nat. Chem. 2009, 1, 37–46.

(56)

Hansgen, D. A.; Vlachos, D. G.; Chen, J. G. Using First Principles to Predict Bimetallic Catalysts for the Ammonia Decomposition Reaction. Nat. Chem. 2010, 2, 484–489.

(57)

Wang, S.; Temel, B.; Shen, J.; Jones, G.; Grabow, L. C.; Studt, F.; Bligaard, T.; Abild-Pedersen, F.; Christensen, C. H.; Nørskov, J. K. Universal Brønsted-Evans-Polanyi Relations for C–C, C–O, C–N, N– O, N–N, and O–O Dissociation Reactions. Catal. Letters 2011, 141, 370–373.

(58)

Ferrin, P.; Simonetti, D.; Kandoi, S.; Kunkes, E.; Dumesic, J. A.; Nørskov, J. K.; Mavrikakis, M. Modeling Ethanol Decomposition on Transition Metals: A Combined Application of Scaling and Brønsted-Evans-Polanyi Relations. J. Am. Chem. Soc. 2009, 131, 5809–5815.

(59)

Viñes, F.; Vojvodic, A.; Abild-Pedersen, F.; Illas, F. Brønsted-Evans-Polanyi Relationship for Transition Metal Carbide and Transition Metal Oxide Surfaces. J. Phys. Chem. C 2013, 117, 4168–4171.

(60)

Loffreda, D.; Delbecq, F.; Vigné, F.; Sautet, P. Fast Prediction of Selectivity in Heterogeneous Catalysis from Extended Brønsted-Evans-Polanyi Relations: A Theoretical Insight. Angew. Chem. Int. Ed. 2009, 48, 8978–8980.

(61)

Santen, R. A. V.; Neurock, M.; Shetty, S. G. Reactivity Theory of Transition-Metal Surfaces: A Brønsted-Evans- Polanyi Linear Activation Energy-Free-Energy Analysis. Chem. Rev. 2010, 110, 2005–2048.

(62)

Bligaard, T.; Nørskov, J. K.; Dahl, S.; Matthiesen, J.; Christensen, C. H.; Sehested, J. The Brønsted-Evans-Polanyi Relation and the Volcano Curve in Heterogeneous Catalysis. J. Catal. 2004, 224, 206–217.

(63)

Medford, A. J.; Vojvodic, A.; Hummelshøj, J. S.; Voss, J.; Abild-Pedersen, F.; Studt, F.; Bligaard, T.; Nilsson, A.; Nørskov, J. K. From the Sabatier Principle to a Predictive Theory of Transition-Metal Heterogeneous Catalysis. J. Catal. 2015, 328, 36–42.

(64)

Chianelli, R. R.; Berhault, G.; Raybaud, P.; Kasztelan, S.; Hafner, J.; Toulhoat, H. Periodic Trends in Hydrodesulfurization: In Support of the Sabatier Principle. Appl. Catal. A Gen. 2002, 227, 83–96.

(65)

Grabow, L. C.; Studt, F.; Abild-Pedersen, F.; Petzold, V.; Kleis, J.; Bligaard, T.; Nørskov, J. K. Descriptor-Based Analysis Applied to HCN Synthesis from NH3 and CH4. Angew. Chem. Int. Ed. 2011, 50, 4601–4605.


30

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

TOC

(8.5 cm × 4.75 cm)


31

C-Methylation of Alcohols, Ketones, and Indoles with Methanol Using

Recommend Documents